Control Unit for Electric Power Steering Apparatus

ABSTRACT

It is an object of the present invention to provide a control unit for an electric power steering apparatus for accurately detecting an abnormality of a steering angle sensor without forming the steering angle sensor as a double system or using an expensive position detection sensor. The object is achieved by calculating a relative steering angle by using inexpensive position detection sensors disposed at a motor of the electric power steering apparatus to output binary values and comparing a change amount of an absolute steering angle detected by the steering angle sensor and a change amount of the calculated relative steering angle with each other to thereby detect the abnormality of the steering angle sensor.

TECHNICAL FIELD

The present invention relates to a control unit for an electric powersteering apparatus for applying a steering assist force by a motor to asteering system of an automobile or a vehicle and especially to acontrol unit for an electric power steering apparatus for detecting anabnormality of a steering angle sensor for detecting a steering angle ofa steering shaft.

BACKGROUND TECHNIQUE

An electric power steering apparatus for applying a steering assistforce to a steering mechanism of an automobile by a rotating force of amotor transmits a driving force of the motor by a transmitting mechanismsuch as a gear and a belt through a reduction gear to apply the steeringassist force to a steering shaft or a rack shaft. A simple structure ofsuch an electric power steering apparatus is shown in FIG. 12 and willbe described with reference to the drawing.

A shaft 102 of a steering wheel 101 is coupled to tie rods 106 ofsteered wheels through a reduction gear 103, universal joints 104 a and104 b, and a pinion rack mechanism 105. The shaft 102 is provided with atorque sensor 107 for detecting steering torque of the steering wheel101 and a motor 108 for assisting a steering force of the steering wheel101 is coupled to the shaft 102 through the reduction gear 103. Themotor of the electric power steering apparatus is controlled by acontrol unit 109 by inputting a torque value T detected by the torquesensor 107, a vehicle speed V detected by a vehicle speed sensor (notshown), a rotation angle of the motor detected by a position detectionsensor 110 for detecting a rotation position of the motor, and further asteering angle θs detected by a steering angle sensor 112 attached tothe reduction gear 103, or the like to the control unit 109. The controlunit 109 is mainly formed of a CPU and performs a motor control by usinga program in the CPU.

The detected steering angle is used for an attitude control of thevehicle or used for controlling the electric power steering apparatus.Therefore, if the steering angle sensor comes into an abnormalcondition, it is unfavorable to use the erroneous steering angledetected by the steering angle sensor for control and therefore, it isnecessary to detect the abnormality of the steering angle sensor withoutdelay. It is conceivable to form the steering angle sensor as a doublesystem, but it increases the cost and thus other various abnormalitydetecting means have been devised.

There is a means for detecting an abnormality of a steering angle sensoras disclosed in Japanese Patent Application Laid-open (JP-A) No.2002-104211, wherein a steering angle estimated from terminal voltage ofa motor and motor current and a steering angle detected by the steeringangle sensor 112 are compared with each other to detect the abnormalityof the steering angle sensor.

There is one disclosed in JP-A No. 2003-252228, wherein a steering angledetected by a steering angle sensor 112 and a steering angle estimatedfrom a motor rotation angle detected by a position detection sensor 110for detecting the rotation position of the motor are compared to eachother to detect an abnormality of the steering angle sensor.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, with the steering angle estimated from the terminal voltage ofthe motor and the motor current, it is difficult to detect the steeringangle with accuracy. Moreover, in order to accurately detect thesteering angle by using the motor position detection sensor, it isnecessary to employ an expensive position detection sensor such as aresolver as the position detection sensor.

The present invention has been made in view of the above-describedcircumstances, and the object of the invention is to provide a controlunit for an electric power steering apparatus for detecting anabnormality of a steering angle sensor by accurately estimating arelative steering angle by using an inexpensive motor position detectionsensor.

MEANS TO SOLVE THE PROBLEMS

The present invention relates to a control unit for an electric powersteering apparatus including a steering angle sensor for detecting asteering angle of a steering shaft and controlled to apply a steeringassist force by a motor to a steering system of a vehicle and, toachieve the above object of the invention, the control unit includes: aplurality of position detection sensors for outputting binary valuesaccording to a rotation position of the motor; relative steering angledetecting means for detecting a relative steering angle of the steeringshaft from binary outputs of the position detection sensors; andabnormality determining means for determining that the steering anglesensor or the relative steering angle detecting means is abnormal when adifference between a change amount of the steering angle and a changeamount of the relative steering angle is greater than a predeterminedvalue to output an abnormality signal.

To further effectively achieve the above object of the invention, thecontrol unit further includes self-diagnostic means for determining anabnormality of the relative steering angle, the control unit determiningthat the steering angle sensor is abnormal when the abnormalitydetermining means outputs the abnormality signal and the self-diagnosticmeans determines that the relative steering angle is normal.

Moreover, to further effectively achieve the above object of theinvention, the control unit further includes steering angle interruptingmeans for interrupting input of the steering angle from the steeringangle sensor, the input of the steering angle output from the steeringangle sensor is interrupted by the steering angle interrupting meanswhen the abnormality determining means outputs the abnormality signaland the motor is controlled without using the steering angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a relationship between a rotation positionof a motor and an output value of a state function when outputs of Hallsensors are input.

FIG. 2 is a drawing showing relationships among the output values of theHall sensors and a state value S_(n).

FIG. 3 is a drawing showing a relationship between a rotating directionof the motor and the state value S_(n).

FIG. 4 is a drawing showing relationships among state values S_(n−1),S_(n) before and after a predetermined time, the rotating direction, andthe abnormality of the rotating direction detection.

FIG. 5 is a drawing showing relationships among state values S_(n−1),S_(n), the rotating direction, and abnormality of the rotating directiondetection with abnormalities of the Hall sensors in view.

FIG. 6 is a control block diagram of the invention.

FIG. 7 is a flow chart of motor rotating direction detection andprocessing for determining the abnormality of rotating directiondetection of the invention.

FIG. 8 is a flow chart for calculating a steering angle.

FIG. 9 is a flow chart for calculating a steering wheel relativesteering angle and a steering velocity of the steering wheel.

FIG. 10 is a control block diagram of abnormality determination.

FIG. 11 is a flow chart of abnormality determination.

FIG. 12 is a block diagram of an electric power steering apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is formed of a relative steering angle estimatingportion for estimating a relative steering angle that is a relativesteering angle by using inexpensive motor position detection sensors andan abnormality determining portion for determining an abnormality bycomparing a steering angle detected from a steering angle sensor and theestimated relative steering angle with each other.

First, a theory of the former relative steering angle estimating portionfor estimating the relative steering angle that is the relative steeringangle by using the inexpensive motor position detection sensors will bedescribed and then an embodiment of the invention will be described.

The relative steering angle estimating portion is further divided into aportion related to a self-diagnostic means for self-determining theabnormality of the position detection sensors for outputting binaryvalues and a portion for estimating the relative steering angle that isthe relative steering angle by using the position detection sensors.

Therefore, a detection principle of the self-diagnostic means fordetermining the abnormality of the position detection sensor foroutputting the binary value will be described first and then a principleof estimation of the relative steering angle by using the output of theposition detection sensor will be described.

A principle in a case in which three Hall sensors HS1, HS2, and HS3 aredisposed at a stat or of a motor 108 as the position detection sensorsfor detecting a position of a rotor of the motor 108 to detect theposition of the rotor will be described below. The Hall sensor taken asan example of the position detection sensor for outputting the binaryvalue is generally available as an inexpensive part. Relationships amongthe binary outputs when the three Hall sensors HS1, HS2, and HS3 aredisposed at an equal interval of 120° at the stator of the motor 108 areshown in FIG. 1. The output of HS1 changes from “0” to “1” or from “1”to “0” at every 180°. The output of HS2 changes from “0” to “1” or from“1” to “0” at every 180° with a 120° phase shift from the output of HS1.The output of HS3 changes from “0” to “1” or from “1” to “0” at every180° with a 240° phase shift from HS1 and with a 120° phase shift fromHS2.

Here, a state function to which the output values of HS1, HS2, and HS3are input is determined. An output value of the state function is in aone-to-one relationship with each rotation position of the motor withoutredundancy. As an example, an expression 1 is utilized as the statefunction. $\begin{matrix}\begin{matrix}{S = {{4 \cdot \left\lbrack {{HS}\quad 3} \right\rbrack} + {2 \cdot \left\lbrack {{HS}\quad 2} \right\rbrack} + \left\lbrack {{HS}\quad 1} \right\rbrack}} \\{= {{2^{2} \cdot \left\lbrack {{HS}\quad 3} \right\rbrack} + {2^{1} \cdot \left\lbrack {{HS}\quad 2} \right\rbrack} + {2^{0} \cdot \left\lbrack {{HS}\quad 1} \right\rbrack}}}\end{matrix} & \left( {{Expression}\quad 1} \right)\end{matrix}$

Here, [HS1], [HS2], and [HS3] are the output values of HS1, HS2, andHS3, respectively, and take on either one of “0” and “1”. The statefunction S is not limited to the above expression 1 and another functionmay be used if an output value S_(n) (hereinafter, referred to as a“state value S_(n)”) of the state function S is in a one-to-onerelationship with the rotation position of the motor without redundancy.

FIG. 1 shows a result of calculation of the state function S of theexpression 1. From FIG. 1, it can be understood that the position of therotor of the motor at every 60° is in a one-to-one relationship with thestate value S_(n). In FIG. 1, a direction of a rightward movement, e.g.,a direction in which the value S_(n) moves from “5”, to “1” and from “1”to “3” is a clockwise rotation (hereinafter, referred to as CW).Reversely, a direction of a leftward movement, e.g., a direction inwhich the value S_(n) moves from “5” to “4” and from “4” to “6” is acounterclockwise rotation (hereinafter, referred to as CCW).

Relationships among the output values of the respective HS and the statevalue S_(n), which is the output value of the state function S, areshown in FIG. 2. In FIG. 2, although “0” and “7” of the state valueS_(n) are not defined as rotation positions, they are listed, becausethey are possible to exist as the output values of the state function S.To put it concretely, when one of the Hall sensors gets out of order andthe output of it is constantly “0” or “1”, “0” and “7” exist as thestate value S_(n)

FIG. 3 is a drawing for making relationships between CW and CCW whichare the rotating directions and the output value of the state valueS_(n) easy to understand and shows the relationship between the rotatingdirection of the motor and the state value S_(n). It can be understoodfrom FIG. 3 that a relationship of movement from a certain state valueS_(n) to another state value S_(n+1) is fixed. For example, if the statevalue S_(n) is “1”, then the value certainly moves to “3” in a case ofthe direction CW while moves to “5” in a case of the direction CCW.Therefore, the state value S_(n) does not directly move from “1” to “2”,“4”, or “6” and such movement is regarded as abnormal.

A relationship of a change of the state value S_(n) from a certain timeto the next time is shown in FIG. 4. FIG. 4 shows a relationship betweena state value S_(n−1) which is an output value of the state function Sat a certain time and the next state value S_(n). In FIG. 4, if thestate value S_(n−1) at a certain time is “1” and the next state valueS_(n) is “3”, the rotating direction is CW and therefore “CW” isindicated in a position of a point of intersection of S_(n−1) and S_(n)in FIG. 4. If the next state value S_(n) is “5”, the rotating directionis CCW and therefore “CCW” is indicated in a position of a point ofintersection of S_(n−1) and S_(n). Here, if S_(n−1) is “1” and S_(n)is“1” this represents a rotation stop in which the motor does not rotateand is at a standstill in the same position and therefore “0” isindicated in a position of a point of intersection of S_(n−1) and S_(n).Incidentally, if S_(n−1) is “1” and S_(n) is “2”, “4”, or “6”, thisrepresents an abnormality and therefore “E” is indicated.

If a failure of the Hall sensor is considered, “0” and “7” of the statevalue S_(n) exist and therefore it can be said that FIG. 5 indicatesevery case. Therefore, “E” is indicated at every intersection with “0”or “7” of the state value S_(n−1) or S_(n) premised on the abnormalityof the Hall sensor.

FIG. 5 means that the rotating direction of the motor and the detectionabnormality can immediately be determined if a state value S_(n) at acertain time and a state value S_(n+1) at the next time are obtained.This relationship is defined as in an expression 2.X=T[S _(n−1) ][S _(n)]  (Expression 2)

According to the expression 2, rotation in the direction CW is output as“1”, rotation in the direction CCW is output as “−1”, a rotation stop isoutput as “0”, and the detection abnormality E is output as “127” as theoutput value X, because the relationship of the rotation in thedirection CW, the rotation in the direction CCW, the rotation stop, orthe detection abnormality is recognized from the relationship in thetable in FIG. 5 if the state value S_(n−1)at a certain time and a statevalue S_(n) at the next time are obtained. Therefore, it is possible toimmediately recognize the rotating direction of the motor and thedetection abnormality (abnormality is indicated in the self-diagnosis)by seeing the output X at one time.

The above is the explanation of the detection principle of theself-diagnostic means of the relative steering angle estimating portion.

Next, a detection principle of the relative steering angle, which is theremaining half of the relative steering angle estimating portion, willbe described below. From FIG. 5, if there is no detection abnormality,it can immediately be recognized that the motor is rotating in thedirection CW, that the motor is rotating in the direction CCW, or thatthe motor is in the stop state of rotation. Therefore, as the value ofthe output X defined in the expression 2, the rotation in the directionCW is output as “1”, the rotation in the direction CCW is output as“−1”, and the stop state of the rotation is output as “0”. If the threeHall sensors are disposed at equal intervals of 120°, “1” represents anamount corresponding to 60°. These angles of 120° and 60° representelectrical angles and hereinafter, angles represent electrical angles.

Therefore, if a relationship from a state value S_(n) at a certain timeto a state value S_(n+1) at the next time is “1”, it can be recognizedthat the motor rotates 60° in the direction CW. Then, if a relationshipof a change from the state value S_(n+1) to a state value S_(n+2) isalso “1”, it can be recognized that the motor further rotates 60° in thedirection CW. Reversely, if a relationship from a state value S_(n) at acertain time to a state value S_(n+1) at the next time is “−1”, it canbe recognized that the motor rotates 60° in the direction CCW. If arelationship from a state value S_(n) at a certain time to a state valueS_(n+1) at the next time is “0”, it means that the motor is not rotatingbut is stopped. Therefore, from the relationships in FIG. 5, therelative rotation position of the motor can be determined by adding theoutput value X, i.e., “1”, “−1”, or “0” representing rotation in thedirection CW, rotation in the direction CCW, or the stop, respectivelyto the previous state and integrating the addition results. In otherwords, an addition may be performed as shown in the following expression(3) to obtain an addition result.C _(n) t=C _(n) t+X  (Expression 3)

In other words, if a result of an addition of the output value X to theprevious addition result Cnt is defined as a new addition result Cnt,addition results are integrated and the relative motor rotation anglecan be calculated.

Next, from the number of times of rotation of the motor, a steeringwheel steering angle A_(n) can be calculated. This steering wheelsteering angle A_(n) is a steering angle used for calculating a relativesteering angle RA of a steering shaft (hereinafter, referred to as asteering wheel relative steering angle RA).

First, the steering wheel steering angle A_(n) is calculated. In orderto calculate the steering wheel steering angle A_(n), a gear ratio of aworm and the like need be considered. This, relationship is differentdepending on the electric power steering apparatus. In a case of athree-phase four-pole motor, for example, the relationship is expressedas in an expression 4.A _(n) =K·C _(n) t+T _(n) /Kt  (Expression 4)

Here, K=60°/2/G. G represents the gear ratio of the worm gear. Thesecond term, T_(n)/Kt represents a torsion angle of a torsion bar andthe torsion angle is also taken into consideration to perform theaddition. T_(n) represents a torque value detected at the same time asthe state value S_(n). and Kt represents a spring constant.

Next, the steering wheel relative steering angle RA is calculated.First, the steering wheel relative steering angle RA can be calculatedas the following expression 5.RA=A _(n) A _(n-m)  (Expression 5)

Wherein, A_(n) represents a steering wheel steering angle at a certaintime and A_(n-m) represents a steering wheel steering angle m stepsbefore the certain time. The steering wheel steering angle A_(n) is therelative steering angle and therefore need not be absolutely accurate.

Also, a steering velocity of the steering wheel Vh is calculated.

In other words, in order to calculate the steering velocity of thesteering wheel Vh, a time t_(m), which has been required for changes ofm steps, is known and therefore the expression 6 may be performed byusing the steering wheel relative steering angle RA and the time t_(m).Vh=RA/t _(m)  (Expression 6)

If t_(m) is set at a time such as 100 ms, for example, the steeringvelocity of the steering wheel Vh can be calculated directly from theexpression 6.

The above are theoretical description of simultaneous detection of theabnormality of the rotating direction detection (self-diagnosis) and therotating direction when the rotating direction detection is normal andtheoretical description of calculation of the steering wheel relativesteering angle RA (relative steering angle RA of the steering shaft).

EMBODIMENT

Next, the preferred embodiment of the invention will be describedspecifically based on the drawings.

The embodiment will be described by dissecting it in the portion forestimating the relative steering angle by using the Hall sensors and theportion for determining the abnormality by comparing the steering angle(absolute steering angle) detected by the steering angle sensor and therelative steering angle obtained by estimation and calculation.

First, the portion for estimating the relative steering angle by usingthe Hall sensors and the self-diagnostic means will be described withreference to the drawings. Control processing which will be describedbelow is performed every predetermined time. The predetermined time is atime required for 1 step from an n step, which is a certain state, to an(n+1) step which is the next state. This predetermined time isdetermined by considering all things such as performance of CPU of acontrol unit and detection velocities of detection sensors.

FIG. 6 is a control block diagram related to the self-diagnostic means Band the relative steering angle detecting means A.

Outputs HS1, HS2, and HS3 of the Hall sensors are input to theself-diagnostic means B and the means B detects the abnormality of therotating direction detection (self-diagnosis) and the rotating directionwhen the rotating direction detection is normal at one time.

A configuration of the self-diagnostic means B includes the Hall sensorsHS1, HS2, and HS3 disposed in the motor, state function calculatingmeans 11 to which the outputs of the sensors are input, and determiningmeans 12 to which an output of the state function calculating means 11is input. The determining means 12 is further consisted of storage means12 ⁻¹ and a determination table 12 ⁻². The storage means 12 ⁻¹ storesthe state value S_(n) which is the output of the state functioncalculating means 11 and outputs the state value S_(n−1), of a state,which is one step before the processed step, to the determination table12 ⁻². The state value S_(n), and the state value S_(n−1) are input tothe determination table 12 ⁻² and the table 12⁻² outputs a determinationvalue X. The determination table 12 ⁻² is a table for determining therotating direction and the abnormality of the rotating directiondetection shown in FIG. 5.

In this configuration of the self-diagnostic means B, operation of theconfiguration will be described with reference to a flow chart in FIG.7. The Hall sensors HS1, HS2, and HS3 which are the position detectionsensors output “0” or “1” which are binary outputs corresponding to therotation position of the motor. The outputs “HS1”, “HS2”, and “HS3” ofthe Hall sensors are input to the state function calculating means 11(step S1).

In this state function calculating means 11, S_(n)=4·“HS3”+2 “HS2”+“HS1”which is the expression 1 is calculated. The state value S_(n), which isa result of this calculation, is input to the determining means 12 (stepS2). Calculation of the state function is performed every predeterminedtime.

The state value S_(n) input to the determining means 12 is input to thestorage means 12 ⁻¹ and the determination table 12 ⁻². First, thestorage means 12 ⁻¹ stores the state value S_(n) (step S3). Then, thestorage means 12 ⁻¹ outputs the state value S_(n−1), which is one stepbefore the processed step, to the determination table 122 ⁻² (step S4).

To the determination table 12 ⁻², the state value S_(n) and the statevalue S_(n−1), which are output values of the state function before andafter the predetermined time, are input (step S5). The determinationtable 12 ⁻² immediately determines a relationship between the statevalue S_(n) and the state value S_(n−1). If the state value S_(n) is “1”and the state value S_(n−1) is “3”, for example, the motor is rotatingin the direction CCW. If the state value S_(n) is “1” and the statevalue S_(n−1) is “5”, the motor is rotating in the direction CW. If thestate value S_(n) is “1” and the state value S_(n−1) is “1”, the motoris not rotating but is stopped. If the state value S_(n) is “1” and thestate value S_(n−1) is “6”, the rotation detection is abnormal.

The output of the determination table 12 ⁻² is output as the outputvalue X of the expression 2. In other words, the table 12 ⁻2 outputs “1”in the case of the CW rotation, “−1” in the case of the CCW rotation,“0”in the case of the rotation stop, and “E” or “127” in the case of theabnormality of the rotation detection (step S6).

The rotating direction of the motor and the abnormality of the rotatingdirection detection can be detected at one time without using aconditional statement by using the table. When the output E of theself-diagnosis detection abnormality is output, in other words, a resultof determination by the self-diagnostic means B that the motor rotatingdirection is abnormal is utilized for identifying normality of thesteering angle sensor as will be described later. Reversely, when theoutput E of the self-diagnosis detection abnormality is not output, aresult of determination by the self-diagnostic means B that the motorrotating direction is normal is utilized for identifying abnormality ofthe steering angle sensor.

Next, an embodiment of calculation of the steering wheel relativesteering angle RA will be described with reference to the control blockdiagram in FIG. 6 and flow charts in FIG. 8 and FIG. 9.

First, the relative steering angle detecting means A is consisted of theself-diagnostic means B including the state function calculating means11 and determining means 12, a relative steering angle counter 13, and arelative steering angle calculating means 14 as shown FIG. 6. In otherwords, from the self-diagnostic means B, the respective state signals ofCW, CCW, and the stop representing the rotating directions are output inaddition to the detection abnormality output E. These signals (CW, CCW,and the stop) representing the rotating directions are input to therelative steering angle counter 13 and the steering wheel steering angleA_(n) is output. Next, the steering wheel steering angle A_(n) is inputto the relative steering angle calculating means 14 and the steeringwheel relative steering angle RA is output.

First, a calculating procedure of the steering wheel steering angleA_(n) by the relative steering angle counter 13 will be described withreference to the flow chart in FIG. 8. First, the CW rotation, the CCWrotation, the rotation stop, which are the rotating directions of themotor, are converted into numerical values. In the present embodiment,the determination table 12 ⁻² carries out detection of the rotatingdirection and conversion of the rotating direction into the numericalvalue at one time. The CW rotation, the CCW rotation, and the rotationstop are converted into “1”, “−1”, and “0” respectively. In other words,X takes on any one of “1”, “−1”, and “0”) step S11). Next, the numericalvalue X continues to be added every predetermined time, i.e., at everystep to calculate the integrated value C_(n)t. In other words, anexpression, Cnt=Cnt+X is performed and, as a result, X is integrated tocalculate the integrated value C_(n)t (step S12).

Next, based on the expression, A_(n)=K·C_(n)t+T_(n)/Kt defined by theexpression 4, the steering wheel steering angle A_(n) is calculated(step S13). Here, the torque T_(n) is a torque value at an n step.Finally, the counter finishes the n step and performs countingcorresponding to the (n+1) step (step S14). The above is the operationof the relative steering angle counter 13.

Next, the steering wheel relative steering angle RA is obtained. Thesteering wheel relative steering angle RA is calculated by the relativesteering angle calculating means 14. The relative steering anglecalculating means 14 performs the expressions 5 in the flow chart inFIG. 9. In other words, the steering wheel relative steering angle RA iscalculated by subtracting a steering angle A_(n−m) in a step m stepsbefore the present n step from the steering angle A_(n) in the present nstep (step S21).

Moreover, if the calculated steering wheel relative steering angle RA isdivided by a time t_(m) required for the m steps, the steering velocityof the steering wheel Vh is calculated (step S22).

The above is the embodiment of the portion for estimating the relativesteering angle by using the Hall sensors. Next, the embodiment of theportion for determining the abnormality of the steering angle sensor byusing the steering angle θs (absolute steering angle) detected by thesteering angle sensor 112 and the steering wheel relative steering angleRA estimated by the above-described method will be described withreference to the control block diagram in FIG. 10 and the flow chart inFIG. 11.

A determination principle of abnormality determination will be describedfirst and then the embodiment will be described with reference to FIGS.10 and 11.

Here, a change amount Δθs of the steering angle θs, which is input tothe abnormality determining means C, is an amount of change from a valueθs0 read in as an initial value by the steering angle sensor 112. Inother words, a relational expression 7 holds.Δθs=θs−θs0  (Expression 7)

Likewise, a change amount ΔRA of the relative steering angle, which isanother input to the abnormality determining means C, is an amount ofchange from a value RAO read in as an initial value of the steeringwheel relative steering angle RA. In other words, a relationalexpression 8 is established.ΔRA=RA−RA0  (Expression 8)

If the steering angle θs which is the absolute steering angle detectedby the steering angle sensor 112 is detected normally and the steeringwheel relative steering angle RA which is the relative steering angleestimated by the relative steering angle detecting means A is calculatednormally, Δθs=ΔRA. If either one of steering angle Δs and steering wheelrelative steering angle RA is abnormal, Δθs=ΔRA does not hold. However,in view of a detection error of the steering angle sensor and acalculation error of the relative steering angle, it is possible todetermine which of the steering angle θs detected by the steering anglesensor and the calculated relative steering angle RA is abnormal bydetermining whether or not |ΔθS−ΔRA|, which is a difference between thechange amount Δθs and the change amount ΔRA, is greater than apredetermined value Δθth as shown in an expression 9.|Δθs−ΔRA|>Δθth  (Expression 9)

Moreover, it is possible to know the abnormality of the calculation ofthe relative steering angle RA by self-diagnosis by the self-diagnosticmeans B and therefore, by adding this self-diagnosis to thedetermination result by the expression 9, the abnormality of thesteering angle sensor 112 can be determined. The above is thedetermination principle of the abnormality determination.

With reference to FIGS. 10 and 11, the embodiment will be described.

The abnormality determining means C is formed of a subtracting means 20,an absolute value means 21, a predetermined value setting means 22, anda comparing means 23.

In FIG. 10, the change amount Les of the steering angle θs, which is theabsolute steering angle detected by the steering angle sensor 112 andthe change amount (relative steering angle) ARA of the steering wheelrelative steering angle RA calculated by the relative steering anglecalculating means A, are input to the subtracting means 20 to calculatethe difference (Δθ−ΔRA) and the absolute value means 21 calculates theabsolute value |ΔθS−ΔRA|.

Next, the predetermined value setting means 22 compares the setpredetermined value Δθth and the absolute value |Δθ−ΔRA| which is theoutput of the absolute value means 21 with each other. If the absolutevalue |Δθs−ΔRA| is smaller than the predetermined value Δθth, it isdetermined that both the steering angle Δθs and steering wheel relativesteering angle RA are normal. Reversely, if the absolute value|Δθs−ΔARA| is greater than the predetermined value Δθth, it isdetermined that either one of the steering angle θs and steering wheelrelative steering angle RA is abnormal and an abnormality signal (ER) isoutput.

Moreover, by combining the signal (E) of the abnormality of theself-diagnostic detection output by the self-diagnostic means B with theabove signal, it is possible to specify which of the steering angle θsand the steering wheel relative steering angle RA is abnormal. Forexample, if the abnormality determining means C determines theabnormality to output the abnormality signal (ER) while theself-diagnostic means B does not output the signal (E) of theabnormality of the self-diagnostic detection means (i.e., if it isdetermined that the self-diagnostic means is normal), it is possible todetermine that detection of the steering angle θs is abnormal. In thiscase, there is an extremely high probability that the steering anglesensor 112 is abnormal.

The above will be described with reference to the flow chart in FIG. 11as follows.

Whether or not initial values of the steering angle θs which is theoutput of the steering angle sensor and the steering wheel relativesteering angle RA have been read in is determined (step S31). If theyhave not been read in (NO), the steering angle θs0, which is the initialvalue from the steering angle sensor, is read in and stored (step S32).Likewise, the initial value RAO of the steering wheel relative steeringangle RA (calculated in step S21) is read in and stored (step S33).

On the other hand, if the initial values have been read in and stored(YES), the processing advances as follows. The steering angle θsdetected by the steering angle sensor in a cycle programmed in the CPUis read in (step S34). Next, the steering wheel relative steering angleRA calculated in step S21 is read in (step S35). The change amountΔθs=θs−θs0 of the steering angle is calculated (step S36). Likewise, thechange amount ΔRA=RA−RA0 of the steering wheel relative steering angleis calculated (step S37).

Whether or not an error between the change amount Δθs of the steeringangle and the change amount ΔRA of the steering wheel relative steeringangle is greater than the predetermined value Δθth is determined (stepS38). If the error is smaller than the predetermined value (NO), boththe steering angle and relative steering angle are normal and thereforethe determination in this cycle is completed.

If the error is greater than the predetermined value (YES), either oneof the steering angles and the steering wheel relative steering angle RAis abnormal. The abnormality signal (ER) is output (step S39). Whetheror not the self-diagnostic detection abnormality signal (E) detected bythe self-diagnostic means B has been output is determined (step S40). Ifthe self-diagnostic detection abnormality signal (E) has been output (ifthe self-diagnostic means determines abnormality) (YES), it is possibleto specify that the steering wheel relative steering angle RA isabnormal (step S41). If the self-diagnostic detection abnormality signal(E) has not been output (if the self-diagnostic means determinesnormality) (NO), it is possible to determine that the steering angle θsis abnormal (step S42).

As described above, it is possible to detect the abnormality of thesteering angle θs including the abnormality of the steering anglesensor. If the abnormality of the steering angle sensor 112 is detected,the steering angle θs is not an input signal absolutely necessary forcontrolling the electric power steering apparatus and therefore it ispossible to carry out relatively better control by interrupting input ofthe steering angle θs with a steering angle interrupting means (notshown) and without using the steering angle θs from the steering anglesensor 112 than by using the steering angle θs which is an erroneoussignal. The steering angle interrupting means is controlled by theabnormality signal (ER). Alternatively, it is also possible to controlthe steering angle interrupting means by using the steering wheelrelative steering angle RA instead of the steering angle θs when thesteering angle sensor 112 is abnormal.

Moreover, if it is impossible to determine which of the steering angleθs and the steering wheel relative steering angle RA is abnormal, it isconceivable that control is carried out control without using thesteering angle θs from the steering angle sensor 112. In other words, ifthe steering angle sensor is abnormal, it maybe better for a driver tocarry out the control without using the steering angle θs than to carryout erroneous control by using the erroneous steering angle θs in somecases.

Although the example in which the Hall sensors are used as the positiondetection sensors for outputting binary values has been described in theembodiment, the position detection sensor is not limited to the Hallsensor or a Hall IC.

As described above, with the invention, it is possible to expect theeffect of accurate detection of the abnormality of the steering anglesensor by using the inexpensive position detection sensors.

POSSIBILITIES OF INDUSTRIAL APPLICATION

With the control unit for the electric power steering apparatusaccording to the invention, by detecting the relative steering angle byusing the inexpensive position detection sensors for outputting thebinary values to compare the relative steering angle and the steeringangle detected by the steering angle sensor with each other, it is firstpossible to detect which is abnormal and to diagnose itself as abnormalin detection of the relative steering angle. Therefore, by adding theresult of the self-diagnosis, it is possible to detect the abnormalityof the steering angle sensor.

If there is a possibility that the steering angle is abnormal, it ispossible to carry out more accurate control by controlling the electricpower steering apparatus without the input signal of the steering anglethan by controlling the electric power steering apparatus by using theerroneous input signal of the steering angle.

1. A control unit for an electric power steering apparatus including asteering angle sensor for detecting a steering angle of a steering shaftand controlled to apply a steering assist force by a motor to a steeringsystem of a vehicle, the control unit comprising: a plurality ofposition detection sensors for outputting binary values according to arotation position of the motor; relative steering angle detecting meansfor detecting a relative steering angle of the steering shaft frombinary outputs of the position detection sensors; and abnormalitydetermining means for determining that the steering angle sensor or therelative steering angle detecting means is abnormal when a differencebetween a change amount of the steering angle and a change amount of therelative steering angle is greater than a predetermined value to outputan abnormality signal.
 2. A control unit for an electric power steeringapparatus according to claim 1, wherein the control unit furthercomprises self-diagnostic means for determining an abnormality of therelative steering angle and the control unit determines that thesteering angle sensor is abnormal when the abnormality determining meansoutputs the abnormality signal and the self-diagnostic means determinesthat the relative steering angle is normal.
 3. A control unit for anelectric power steering apparatus according to claim 1, the control unitfurther comprising steering angle interrupting means for interruptinginput of the steering angle from the steering angle sensor, wherein theinput of the steering angle output from the steering angle sensor isinterrupted by the steering angle interrupting means when theabnormality determining means outputs the abnormality signal and themotor is controlled without using the steering angle.
 4. A control unitfor an electric power steering apparatus according to claim 2, thecontrol unit further comprising steering angle interrupting means forinterrupting input of the steering angle from the steering angle sensor,wherein the input of the steering angle output from the steering anglesensor is interrupted by the steering angle interrupting means when theabnormality determining means outputs the abnormality signal and themotor is controlled without using the steering angle.